Spark ignited (SI) internal combustion (IC) engines operated with gaseous fuels produce small amounts of undesirable chemical compounds in the combustion chamber, compounds which are exhausted from the engine at high temperatures (800°-1250° F.). For fuels composed primarily of methane and other light hydrocarbons, the commonly regulated chemicals are nitrogen oxides (NO, NO2, or generally NOx) and carbon monoxide (CO). Nitrogen oxides are formed when nitrogen (N2), a major component of air, reacts with oxygen (O2), another major component of air, when both are exposed to high temperatures and pressures in an engine combustion chamber. Carbon monoxide, on the other hand, is the consequence of failure of the fuel to completely react with oxygen, resulting in the formation of carbon dioxide (CO2). CO and NOx are problematic pollutants inasmuch as their regulated values are in many geographical regions set at or below the limits of current technology.
In strictly regulated regions, current practice to control the emissions from SI/IC engines fueled by methane-rich fuels (natural gas, bio-fuels, landfill gas, etc.), is to install systems in the engine exhaust ducting to eliminate, to the extent required by regulations, such chemicals. For smaller engines (less than 1000 bhp), the common aftertreatment system is a single stage catalyst. In these small systems, the products of combustion exiting the engine are forced through a catalyst monolith (honeycomb structure with precious metal coating) which facilitates the desirable oxidation and reduction reactions:NOx yields N2+O2 CO+O2 yields CO2 
The nitrogen oxides are reduced to gaseous nitrogen (N2) and oxygen (O2), both benign, while the carbon monoxide (CO) is completely oxidized, forming carbon dioxide (CO2), likewise non-harmful and unregulated.
Current catalyst-based emissions systems rely on very accurate control of engine operating parameters to maximize the conversion efficiency of the reactions noted above. Specifically, the simultaneous elimination of NOx and CO through such reactions in a catalytic converter requires a precise operating window of the engine combustion process relative to the mixture of air and fuel. This is depicted in FIG. 1 for a typical SI/IC engine. As shown, rich mixtures result in low NOx out of the catalyst but high CO, while lean mixtures result in low CO, but high NOx. From FIG. 1, it is evident that simultaneous cleanup of NOx and CO requires that the engine air/fuel ratio (AFR) be precisely controlled in the narrow region around the stoichiometric air/fuel ratio. Compliance of both regulated pollutants can only be maintained when the combustion stoichiometry is maintained within points A and B of FIG. 1. The acceptable combustion mixture, to achieve increasingly strict emissions standards, requires that the engine air/fuel ratio be controlled within narrow limits.
Referring still to FIG. 1, there is depicted typical engine emissions as a function of AFR from a SI/IC engine equipped with a single or multiple three-way catalyst (TWC). Meeting the regulated limits for CO and NOx requires that engine AFR be maintained between points A and B of FIG. 1, a band approximately representing the stoichiometric AFR.
Stationary SI/IC engines operating in most applications in the U.S. and elsewhere are highly regulated relative to allowable CO and NOx emissions, which are becoming increasingly controlled. Most notably, the California Air Resource Board (CARB) now recommends limits of 0.07 lb/MWh NOx and 0.1 lb/MWh CO as part of their 2007 standard for Combined Heat and Power (CHP) applications. Applying a heat recovery credit for maintaining a minimum 60% overall system efficiency and assuming a 27% electrical efficiency, the emissions limits stated in terms of actual concentration in the exhaust gas are 3.7 PPM NOx and 8.9 PPM CO. As used herein, “PPM” means parts per million by volume corrected to a standard air dilution factor (15% oxygen equivalent). The area of Southern California under the jurisdiction of the South Coast Air Quality Management District (SCAQMD) has adopted the “CARB 2007” standard for NOx, while restricting CO emissions to a value close to the CARB limit. Other regions in California are likewise adopting similar standards, while other regions of the country are phasing in regulations approaching the CARB 2007 standards (MA, NY, and NJ, for example).
Compliance with the newer standards requires extremely high conversion efficiency in the catalyst for both CO and NOx. Extra-large conversion monoliths are needed in addition to extreme precision in controlling the air/fuel mixture.
FIG. 2 depicts the steady-state AFR control precision required for a standard engine (e.g., model TecoDrive 7400) utilizing a TWC system sized to conform to CARB 2007, as indicated by a pre-catalyst narrow-band heated exhaust gas oxygen sensor millivolt (mV) output that the AFR controller maintains via steady-state (non-dithering) AFR control. As shown in FIG. 2 the engine combustion mixture (air to fuel ratio) is acceptable for catalyst performance to regulated limits only when the signal from a standard lambda sensor in the exhaust duct is maintained between 680 and 694 mV. Above this range, the CO concentration exiting the catalyst exceeds the SCAQMD limit of 8.9 PPM, while below this range the NOx will rapidly exceed the 3.7 PPM limit. Limits shown in FIG. 2 are those of CARB 2007 with a credit for engine heat recovery, such that 60% of the fuel's heat content is purposefully used as electric power or recovered thermal energy. In order to maintain compliance, combustion air to fuel mixture must be maintained within the 14 mV window for the example shown.
A possible method for expanding the control window for engine operation to attain acceptable emissions from both CO and NOx, is to modify the system such that two stages of catalyst systems are used, each operating in distinctly different chemical atmospheres. Early catalyst systems commonly used a two-stage design with inter-stage air injection. In this era, single purpose catalyst monoliths-oxidation or reduction, but not both, were employed. Later, as multi-purpose, single stage catalysts (TWC) were developed, these became the dominant style. The early two-stage systems were employed in stationary gaseous fueled SI/IC engines with success but under far less strict standards. Presumably, the NOx reformation problems encountered with the two-stage systems were present in the earlier era, but were inconsequential relative to the regulated limits at that time.
FIG. 3 depicts the above-described arrangement. As shown, two catalyst stages are plumbed into an exhaust system in series. Air is pumped into the exhaust stream between stages one (CAT 1) and stage 2 (CAT 2) and mixed thoroughly. The engine air-to-fuel ratio is maintained so as to facilitate effective NOx removal in the first stage. The air injected into the exhaust results in an oxidizing environment at the second catalyst stage biased towards the oxidation of CO to CO2, even if the engine AFR is outside the acceptable operating window on the rich side, a highly significant benefit.
Tests utilizing the two-stage system demonstrated that the two-stage strategy with air injection was not only ineffective, but actually detrimental to catalyst performance. NOx emissions from the two-stage system were found to be generally higher than a single-stage system of comparable size and catalyst material loading. This surprising result indicated that a mechanism exists such that NOx is formed in the second stage, made possible by the oxygen rich environment, coupled also with conditions conducive to chemical reaction, i.e., high temperature and an abundance of a catalytic material.
FIG. 9 is a block diagram of a system 900 for treating exhaust from a lean-burn internal combustion engine 910 according to the prior art. The system 900 includes an oxidation catalytic converter 920, a selective catalytic reduction converter (herein “SCR”) 930, and an ammonia slip catalyst 940. In operation, a lean-burn engine generates an exhaust stream 915 that includes carbon monoxide and nitrous oxide (NOx) compounds. The exhaust stream 915 passes across an oxidation catalyst 920 that is configured to oxide carbon monoxide (CO) in the exhaust stream to carbon dioxide (CO2), which is less harmful to the environment. The exhaust stream 915 then passes into the SCR 930, which uses ammonia (NH3) or urea (CH4N2O) as a reducing agent. The reducing agent reacts with the NOx compounds to form nitrogen (N2) and water (H2O). While ideally the reducing agent would fully react in the SCR 930, a fraction of the reducing agent does not react in the SCR 930. This fraction of unreacted reducing agent remains in the exhaust output from the SCR 930. This issue is generally referred to as ammonia slip. To address ammonia slip, the exhaust passes over an ammonia slip catalyst 940, which is configured to eliminate at least a portion of the unreacted ammonia present in the exhaust stream. Some catalysts currently available (e.g., from Johnson Matthey and MIRATECH) lack the features and capabilities of the presently disclosed systems.
There is a therefore a need for alternative catalysts to address ammonia slip.